hi firmgirl,
i haven't been taking yoga all that long, but my teacher has always told us that it's ok to modify poses to be more comfortable.
one pose he has us do that's good for the lower back is to lay on the floor with knees bent and feet flat on the floor. then find where your back arches off the floor (you can use your hand or just mentally find the arch) and press that area down flat so that the whole of your back is in contact with the floor. you have to tip your hips forward to do this, they may even come up off the floor.
it feels really wonderful when my lower back aches.
kristin